This invention relates to selected carbinol-containing polyimides capable of self-crosslinking at low temperatures. It also relates to the process for their preparation and their use as adhesives to form crosslinked products.
Polyimides are synthetic organic resins characterized by repeating imide linkages in the polymer chain which may or may not be terminated with polymerizable or inert (i.e. non-polymerizable) chemical groups. They are available in both linear and crosslinked forms and are noted for their outstanding chemical and physical properties, particularly their high temperature oxidative stability and strength. In addition to their use as adhesives and molded articles, they may be used as precured films and fibers, curable enamels, and laminating resins.
Most polyimides, especially the preferred aromatic polyimides, are extremely difficult to process due to their insolubility and extremely high softening points. Attempts to improve their processability have included the introduction of aliphatic segments into the otherwise aromatic polymers and the introduction of flexibilizing moieties such as alkylene, ether, or thio bridges into the polymer chain.
The most common method of insuring processability is to apply the polymer in the form of the polyamic acid intermediate. However, during curing to the fully or partially imidized resin, an appreciable amount of a volatile by-product (e.g. water, alcohol, or mixtures thereof depending upon the starting tetracarboxylic acid compound) is formed. This leads to the formation of voids when the resin is used as an adhesive between non-porous substrates or for forming molded articles.
One method of overcoming these disadvantages is the use of low molecular weight polyimides end-capped with polymerizable groups such as unsaturated groups. These end-capped polyimides can be subsequently cured to void-free, higher molecular weight resins since no volatile by-product is formed. Polyimides end-capped with 3,6-endomethylene-1,2,3,6-tetrahydrophthalimide and 3-ethynylphenyl groups are respectively disclosed in U.S. Pat. No. 3,528,950 issued Sept. 15, 1970 to H. R. Lubowitz and U.S. Pat. No. 3,879,349 issued Apr. 22, 1975 to N. Billow et al. However, the polyimides terminated with these groups require high cure temperatures, i.e. within the range of about 200.degree.-350.degree. C., to effect further polymerization.
Another method involves the use of polyimide compositions having increased solubility in various organic solvents. Such polyimides are disclosed in European patent application No. 82401866.7 filed Oct. 11, 1982 and published under the No. 0 077 718 on Apr. 27, 1983. The polyimides are prepared by reacting an aromatic diamine with a tetraester or with a diester-diacid derived from benzhydrol-3,3',4,4'-tetracarboxylic acid. The reaction is continued until the resultant carbinol-containing polyimide has an intrinsic viscosity of 0.1-2 dl/g (measured at 30.degree. C. and a concentration of 5 g./l in N-methylpyrrolidone). Preferably approximately equimolar amounts of the aromatic diamine and tetra-ester or diester-diacid are dissolved in an appropriate solvent. It is possible, however, to use an excess of either monomer (generally less than 50 mole % and preferably 0-20 mole %). The reaction mixture formed by the solvent and the reagents is then heated at a temperature of 80.degree. C. or above, preferably 100.degree.-150.degree. C., until the composition reaches the desired viscosity. The degree of imidization is between 50 and 99.9%, with the remaining groups being amide-acid and/or amide-ester groups. For most applications fully imidized polymers with no oxidizable terminal functional groups, especially amine groups, are preferred. These polymers possess maximum resistance to heat and oxidation and release less volatile by-product during use.
The polyimide solutions of the above European patent application are stated to be useful in the preparation of adhesive films. They, however, suffer from the same disadvantage as the end-capped polyimides, i.e. they require high cure temperatures or the addition of a crosslinking agent such as epoxides, isocyanates, alkyl titanates, anhydrides and halides of carboxylic acids, and silicones. The curing temperature typically used was 1 hr. at 100.degree. C., then 2 hours at 200.degree. C., followed by removal of the film from the support and further heat treatment for 20 min. at 300.degree. C. (see Example 3 of the European patent application). If the crosslinking agent is used, it is added in amounts up to 50%. Generally it is mixed with the polyimide solution before use and the crosslinking reaction occurs during the final heating at about 100.degree.-200.degree. C. (see Examples 18 and 19).
There is a need for polyimides with good adhesive properties which can be cured at a low temperature without the use of crosslinkable compounds. The use of high cure temperatures has a significant drawback - namely, the requirement that the substrate be exposed to temperatures of 200.degree.-300.degree. C. to crosslink the resin. Many semi-conductor devices cannot accept these temperatures, and this is an end use area where polyimides are extensively used because of their outstanding properties (i.e. low outgassing and ability to immobilize ions).
The present invention provides selected carbinol-containing polyimides that self-crosslink at low temperatures. It also provides a process for their preparation by the reaction of an excess of a 3,3',4,4'-benzhydroltetracarboxylic acid compound with a polyfunctional primary amine. It further provides adhesives, coatings and molded articles based on the low temperature crosslinkable or crosslinked polyimides.